The invention described herein relates generally to semiconductor metrology of a specimen, such as a wafer or reticle. More specifically, it relates to metrology that employs detection of an optical signal from a periodic target.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrated circuits using semiconductor materials which are layered and patterned onto a substrate, such as silicon. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the device must comply with rigorous specification requirements prior to shipment of the device to the end users or customers.
Optical metrology techniques (such as spectroscopic ellipsometry, scatterometry, reflectometry, etc.) provide powerful, high-throughput capabilities for measuring critical dimensions, overlay, film thicknesses, composition, and other parameters of nanoscale structures. The measurements are often performed on targets that are in the form of repeating periodic structures, e.g., gratings. These gratings usually represent the actual device geometric and material structure or an intermediate design during device fabrication.
Although some optical metrology techniques for analyzing periodic targets work well for certain applications, it would be beneficial to provide improved mechanisms for performing metrology for periodic targets.